1. Field of the Invention
The present invention relates to an illumination device used as a backlight of a display apparatus and to a display apparatus including the same. This invention particularly relates to an illumination device and a display apparatus that can provide improved color purity in a color display.
2. Description of the Related Art
In recent years, as a display apparatus for a television receiver or the like, liquid crystal display apparatuses characterized by, for example, being reduced in power consumption, thickness and weight have found widespread use. A liquid crystal display element per se does not emit light and thus is a so-called non-light-emitting type display element. Therefore, for example, on one principal surface of the liquid crystal display element, a plane light-emitting type illumination device (so-called backlight) is provided.
Backlights are classified roughly into a direct type and a sidelight (referred to also as “edge-light”) type depending on an arrangement of a light source with respect to a liquid crystal display element. A direct type backlight has a configuration in which a light source is disposed on a rear surface side of a liquid crystal display element, and a diffusing plate, a prism sheet and the like are disposed between the light source and the liquid crystal display element so that uniform plane-shaped light is made incident on an entire rear surface of the liquid crystal display element. Such a direct type backlight has been used suitably in, for example, a large-screen liquid crystal display apparatus for a television receiver.
As a conventional light source for a backlight, a cold cathode fluorescent tube (CCFT) has been in common use. Further, with the recent advancement in development of a light-emitting diode (LED) having higher color reproducibility than a cold cathode fluorescent tube, a LED also has been used suitably as a light source for a backlight.
Furthermore, conventionally, a color display has been realized by color filters of three colors of RGB that are provided so as to correspond to pixels of a liquid crystal display element. FIG. 16 is a schematic diagram showing a structure of an active matrix substrate in a conventional active matrix type liquid crystal display element, in which each pixel is shown with a color of color filters corresponding thereto. As shown in FIG. 16, the active matrix substrate includes scanning lines GL and data lines DL that are arranged in a matrix form, a TFT 101 that is disposed at each of intersections of the scanning lines GL and the data lines DL, and a pixel electrode 102 that is connected to a drain electrode of the TFT 101. On an opposing substrate (not shown) opposed to this active matrix substrate, color filter layers of three colors of RGB are formed in stripes. Thus, as shown in FIG. 16, all of pixels in one column connected commonly to each of the data lines DL display one of the colors of RGB. For example, in FIG. 16, all of pixels connected to the data line DL1 display red (R).
In the active matrix type liquid crystal display element configured as above, when a gate pulse (selective voltage) is applied sequentially to the scanning lines GL1, GL2, GL3, GL4, . . . , each of the TFTs 101 connected to one of the scanning lines GL, to which the gate pulse has just been applied, is brought to an ON state, and a value of a gradation voltage that has been applied to a corresponding one of the data lines DL at that point in time is written into the each of the TFTs 101. Consequently, a potential of the pixel electrode 102 connected to a drain electrode of the each of the TFTs 101 becomes equal to the value of the gradation voltage of the corresponding one of the data lines DL. As a result of this, an orientation state of liquid crystals interposed between the pixel electrode 102 and an opposing electrode changes in accordance with the value of the gradation voltage, and thus a gradation display of the pixel is realized. On the other hand, during a time period in which a non-selective voltage is applied to the scanning lines GL, the TFTs 101 are brought to an OFF state, so that the potential of the pixel electrode 102 is maintained at a value of a potential applied thereto at the time of writing.
As described above, in the conventional liquid crystal display element, the color filters of three colors of RGB are arranged in an orderly manner, and while the scanning lines GL are selected sequentially in one frame time period, a gradation voltage of a desired value is applied to each of pixels that correspond to each of the colors of RGB from a corresponding one of the data lines DL, thereby realizing a color display.
As a CCFT used as a light source for a backlight of the above-described conventional liquid crystal display element that performs a color display, a three-wavelength tube or a four-wavelength tube is in general use. The three-wavelength tube is a fluorescent tube having wavelengths of red (R), green (G), and blue (B), and the four-wavelength tube is a fluorescent tube having wavelengths of red, green, blue, and deep red. In the case of the three-wavelength tube, red, green, and blue phosphors are sealed in the tube. In the case of the four-wavelength tube, red, green, blue, and deep red phosphors are sealed in the tube. In either of these cases, at the time of lighting, mixing of light of the respective wavelengths occurs, so that the liquid crystal display element is irradiated with the light that is light (white light) having an emission spectrum in all wavelength regions. Further, in the case where a LED is used as a light source for a backlight, a prism sheet, a diffusing plate and the like are used to mix the respective colors of light outputted from a red LED, a green LED, and a blue LED (a white LED further may be used) so as to form uniform white light, with which the liquid crystal display element then is irradiated.
The following describes a problem with the case where a light source having wavelength regions of the respective colors of red, green, and blue is used as a light source for a backlight.
FIG. 17 is a spectrum diagram showing spectral transmission characteristics of color filters of three colors of RGB. As shown in FIG. 17, the respective spectral transmission spectra of the blue color filter and the green color filter overlap in an area defined by a range of about 470 nm to 570 nm. Further, the respective spectral transmission spectra of the green color filter and the red color filter overlap in an area defined by a range of about 575 nm to 625 nm. Because of this, in the case of using a light source for a backlight having an emission spectrum in all wavelength regions, color mixing occurs in these areas in which the respective spectral transmission spectra overlap, resulting in deterioration in color purity, which has been disadvantageous.
For example, FIG. 18A shows an emission spectrum of a three-wavelength tube, FIG. 18B shows a spectral transmission characteristic of a red color filter in the case where this three-wavelength tube is used as a light source for a backlight, FIG. 18C shows a spectral transmission characteristic of a green color filter in the case where this three-wavelength tube is used as the light source for the backlight, and FIG. 18D shows a spectral transmission characteristic of a blue color filter in the case where this three-wavelength tube is used as the light source for the backlight.
As can be seen from FIG. 18C, a spectral transmission curve of the green color filter partially overlaps a wavelength region of blue. This means that a blue component is mixed into a pixel that is to be displayed in green. Further, as can be seen from FIG. 18D, a spectral transmission curve of the blue color filter also partially overlaps a wavelength region of green. This means that a green component is mixed into a pixel that is to be displayed in blue. Such a color mixing phenomenon occurs also in the case of using a four-wavelength tube as a light source for a backlight and has been a cause of deterioration in color purity.
Conventionally, in order to obtain improved color purity, a driving method (so-called field sequential driving) has been proposed in which LEDs of three colors of RGB are used as light sources for a backlight with respect to a liquid crystal display element including color filters of three colors of RGB, and the LEDs of the respective colors are caused to blink sequentially so that an image of red alone, an image of green alone, and an image of blue alone are displayed in order in one frame (see JP 2003-271100).
However, in the above-described configuration according to the conventional technique, when a frame rate is increased such as in the case where a moving picture display of a high-resolution image is performed, a problem arises that the field sequential driving in which a display is performed in such a manner that one frame is divided into three colors hardly can be performed. Particularly, in the case of a liquid crystal display apparatus, at least presently, a response speed of liquid crystals is not so high as to be sufficient, rendering it almost impossible to realize a high quality moving picture display by the field sequential driving.